Just how much does adding air barrier to a knee wall insulation increase it's performance?

I know we have a lot of scientifically minded people here and I bet someone has access to a reliable test data showing the answer to the question that's been on my mind for years:

When we add Vapor-permeable Air Barrier to those knee walls just how much exactly that increases the performance of that insulation?

I've had hard time finding results of actual test showing how effective this is. 

I am not doubting it's effectiveness, I would just like to quantify it. 

Please share if you know it

Views: 5245

Reply to This

Replies to This Discussion

Honest Building Science question on this topic: If the drywall of the knee wall is well sealed (meaning outlets, top and bottom plates, etc) as well as the floor joist below, if there are fiberglass batts in the knee wall, is is really that important that whatever covers the fiberglass (or other insulation material like net and blow cellulose) is air tight? 

That's a really long way to ask if one air barrier is good enough. Thoughts?

Covering the kneewall back serves 3 purposes.  1) it prevents the batts from falling off later on 2) it forces the batt to contact the drywall 3) it prevents wind washing.

This is why it is very hard to measure the effect of covering the kneewall -if the batts are very well fastened then low potential for benefit #1,  if the batts are making good contact already, then no benefit #2, and  if the area is back in a corner where there is no wind washing, no benefit #3.  So the effect depends on where on the wall you are looking.

Our viewpoint is that as long as you are putting something up, why not give it some R-value so you provide 1, 2 and 3, and also 4) reduced thermal bridging.  That is why we use 1/2" styrofoam if the customer will pay for it, but always 1/4" styrofoam at least.

Ed Minch

"When we add Vapor-permeable Air Barrier to those knee walls just how much exactly that increases the performance of that insulation?"

The original post by Jack is an excellent question, asking if anyone knows of actual testing to quantify the energy savings resulting from adding a "vapor-permeable Air Barrier" to those knee walls. The only post that came close to answering the question is the graph of 'Convective Wind Washing" posted by Alexander, from a presentation by Steve Easley. All of the other posts were conjecture or statements of common facts. The posts about "The Code" were not applicable because it does not specify that you air seal the exterior of the wall. 

I am interested in the answer to the original question, if anyone has found any actual test results. I am also interested in the effectiveness of insulating the roof, rather than the knee wall and floor, if there has been any scientific testing of that. I am interested in knowing the effect of roof slope on rate of heat loss by radiation. At what point does a roof become a wall? I have encountered this question when dealing with the knee walls of an A-frame house.

I am looking for scientific studies on these questions, to help quantify energy savings and to help guide us in deciding the cost effectiveness of our "prescriptions".

I am not interested in conjectures.

thanks

Brad, my belief is that "it depends" is so much a factor here that a study would be meaningless.  The extremes would be if the batt has fallen at one end, and would be insulation properly contacting the interior drywall and no wind washing at the other.  And you will see these extremes in almost every attic.

I believe this could be there reason there are no studies. 

Wow, guys! I am quite surprised to see how much this topic has stirred. 

That's good, I hope this is the case where out of a group conversation and a common search some clarity comes up. So far it hasn't cleared that much, I agree with Brad that a lot of debate moved away from the original subject. 

My understanding of the scientific basis of my initial question is this: 

The main benefit of an air barrier on the back of the insulation is to prevent the movement of air inside of the insulation. Most of it in my understanding is caused by difference in density in the air inside that insulation. It's pretty much the same convective air movement that would have occurred there if there were no insulation, just in slow-motion. 

So the question is just how much does adding an air barrier helps for this. I did not mean to discuss support for insulation, sealing up the openings under the knee walls and tightness of the wall itself,  that's a given.

RSS

Featured Forum Discussions

What causes a temperature plane in a home

Started by Energy Wise Solutions in HVAC. Last reply by Peter Krych on Friday. 4 Replies

Velocity Pressure Testing

Started by Horace Douglas Hunt, Jr. in General Forum. Last reply by Horace Douglas Hunt, Jr. Apr 15. 2 Replies

Videos

  • Add Videos
  • View All

Latest Activity

Profile IconSharon Block, Travis Lundberg, Ryan & Shannon Coon and 2 more joined Home Energy Pros
7 hours ago
Gary Reed added a discussion to the group Job Board
Thumbnail

HOME ENERGY ADVISORS WANTED (NEW YORK STATE: Saratoga & Glens Falls Region)

We are currently seeking experienced HOME ENERGY ADVISEOS to join the Jack Hall Plumbing &…See More
15 hours ago
Profile IconGary Reed and Kurt Shafer joined Diane Chojnowski's group
Thumbnail

Job Board

This group is for posting jobs related to all aspects of the home performance industry including…See More
16 hours ago
Ron Sarrick liked Energy Wise Solutions's discussion What causes a temperature plane in a home
17 hours ago
Kurt Shafer added a discussion to the group Job Board
Thumbnail

Installers for Whole House Fans in Various Cities

Invisco Whole House Fan Company in Temecula CA sells the highest performance fans in history. The…See More
19 hours ago
Kurt Shafer posted a blog post

First Rooftop Whole House Fan for Homes without Attics

Eichler was one of the most famous Mid Century Modern home builders in the 50s and 60s. His homes…See More
19 hours ago
Travis Lundberg replied to angela stanzione's discussion Used Weatherization and auditing equipment for sale in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Do you still happen to have a blower door fan, frame and fabric still for sale?  If so please…"
19 hours ago
Profile IconTravis Lundberg and Alana Barnett joined allen p tanner's group
Thumbnail

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
19 hours ago

Home Energy Pros

Welcome to Home Energy Pros – the unique digital community by and for those who work in the home energy performance arena.

Home Energy Pros was founded by the developers of Home Energy Saver Pro (supported by the U.S. Department of Energy) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network. Please honor our Guidelines

© 2017   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service